selftests/cpufreq: Rename DEBUG_PI_LIST to DEBUG_PLIST

DEBUG_PI_LIST was renamed to DEBUG_PLIST since
8e18faeac3 ("lib/plist: rename DEBUG_PI_LIST to DEBUG_PLIST")

- It's not reasonable to keep the deprecated configs.
- configs under kselftests are recommended by corresponding tests.
So if some configs are missing, it will impact the testing results

CC: "Rafael J. Wysocki" <[e-mail]>
CC: Viresh Kumar <[e-mail]>
CC: [e-mail]
Signed-off-by: Li Zhijian <[e-mail]>
Acked-by: Viresh Kumar <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/cpufreq/config

···
 CONFIG_CPU_FREQ_GOV_CONSERVATIVE=y
 CONFIG_CPU_FREQ_GOV_SCHEDUTIL=y
 CONFIG_DEBUG_RT_MUTEXES=y
-CONFIG_DEBUG_PI_LIST=y
+CONFIG_DEBUG_PLIST=y
 CONFIG_DEBUG_SPINLOCK=y
 CONFIG_DEBUG_MUTEXES=y
 CONFIG_DEBUG_LOCK_ALLOC=y